Heretofore, information recording media wherein recording and readout of information are carried out by irradiating a rotating disc-shaped information recording medium with a laser beam have been known. As recording layers in these information media, those wherein metals having low melting points and dielectric substances are used are proposed. However, these recording layers have disadvantages such as poor preservability, low separating ability, low recording density and high manufacturing cost. Recently, it has been proposed and practiced that dye films whose physical properties may be changed with light of relatively long wavelength are used in recording layers. However, dyes which have absorption bands in long wavelength generally have such problems that they are low in stability against heat and light. Thus, it is the present state of the art that recording layers having recording characteristics which are stable over a long period have not yet been developed.
On the other hand, compact disc (CD) has been widely and practically used for readout of audio such as music.
Compact disc is generally composed of a plastic disc-shaped transparent support wherein pits containing EFM (Eight to Fourteen Modulation) digital audio signal information such as CD format signal have been formed in advance, a reflective thin film made of Al, etc. and a protective film, provided thereon respectively. Readout of the information from CD is carried out by irradiation of the optical disc with a laser beam. CD format signal or the like is read out by change in reflectance owing to the presence or absence of pits.
CD is required based on CD standard to have a recording time of a maximum 74 minutes in the range of a signal surface inside diameter of 45 mm and a signal surface outside diameter of 116 mm and in a pit width of 0.8 micrometer and a track pitch of 1.6 micrometers when recording is carried out by rotating the CD at a constant linear velocity of 1.2 to 1.4 m/sec. Heretofore, such compact discs for audio have been those only for readout wherein pits had been formed in advance on the support, and thus have had a disadvantage that recording, editing and the like of information cannot be made. Therefore, development of DRAW (Direct Read After Write, writable) type CD has been desired.
Further, DRAW type optical disc for CD-ROM (Read Only Memory) or CD-I (Interactive) has been desired in files such as various documents, data and still pictures.
However, since writable optical information recording media having as a recording layer a dye thin film which have hitherto been known have a low reflectance for the dye thin film, even if information could be recorded therein, the information could not be read out by commercially available CD players in which a high reflectance is required.
As for optical disc for recording CD format signal, there have been proposed an optical disc which has a recording layer comprising a dye, a dye composition or the like and wherein recording of CD format signals is made, and a method for recording optical information using the optical disc (J.P. KOKAI Nos. 61-237239, 61-239443, 62-14344 and 62-14345). However, these optical discs have been practically insufficient in recording sensitivity, readout sensitivity or the like.